Rationale: The Sm₂O₃-Y₂O₃-HfO₂ system holds promise for applications in the sphere of high-temperature technologies, particularly the development of ultra-high-temperature ceramics. However, the reliability of refractory materials is dependent on the possible selective vaporization of their components leading to changes in their physicochemical properties. Thus, information about vaporization processes and thermodynamic properties of ceramics based on the Sm₂O₃-Y₂O₃-HfO₂ system may be of importance for the production of high-temperature materials as well as for the prediction of the physicochemical properties of ultra-high-temperature ceramics. Methods: The Knudsen effusion mass spectrometric method was used, with an MS-1301 magnetic mass spectrometer equipped with a tungsten twin effusion cell used to examine the samples in the Sm₂O₃-Y₂O₃-HfO₂ system. Electron ionization of vapor species effusing from the cell was carried out at an ionization energy of 25 eV. Results: It was shown that at a temperature of 2373 K, selective vaporization of Sm₂O₃ and Y₂O₃ occurred in the samples of the Sm₂O₃-Y₂O₃-HfO₂ system, with the main vapor species being SmO, Sm, YO, and O. The partial pressures of these vapor species were obtained by the ion current comparison method. The Sm₂O₃ activities in the Sm₂O₃-Y₂O₃-HfO₂ system were determined and allowed the evaluation of the excess Gibbs energies at 2373 K. The direction of change of the condensed phase of the samples because of selective vaporization of the components was examined. Conclusions: The Sm₂O₃-Y₂O₃-HfO₂ system was characterized by negative deviations from the ideal behavior at 2373 K. The excess Gibbs energies evaluated in the present study were approximated using the Redlich-Kister representation and visualized in the form of curves of constant values in the concentration triangle. The data obtained in the Sm₂O₃-Y₂O₃-HfO₂ system were optimized using the Barker-Guggenheim theory of associated solutions.